On the surface, there would appear to be few similarities between Lincoln, the rail-splitting Kentuckian who became president, and Darwin, the Victorian Englishman of good and gentle breeding. Their great achievements in life were distinctively different.

But Lincoln and Darwin shared at least one deep commonality: Both despised and opposed slavery. We know, of course, where this antipathy took Lincoln – the Emancipation Proclamation in 1862 that declared all slaves of the Confederacy free. How Darwin's hatred of slavery changed his life and legacy is less known and more surprising.

In “Darwin's Sacred Cause,” authors Adrian Desmond and James Moore make the case that Darwin's fervent opposition to slavery significantly fueled and formed his enduring theories about natural selection and human evolution.

But it didn't happen easily.

“Darwin was the most gentlemanly gentleman anyone had ever met,” the authors write. “He was diffident, afraid to ruffle feathers, at home with the conservative Anglican dons, wanting only his own quiet vicarage lifestyle, away from urban tumult and religious harangues.”

Even so, he could not ignore the great moral debate and movement of his day – the effort in Great Britain to abolish slavery. As a boy, Darwin had been imbued with anti-slavery values by his generally progressive family, which included grandfather Erasmus Darwin, a respected physician-philosopher and friend to such luminaries as Joseph Priestley (credited with discovering oxygen) and James Watt, whose steam engines powered the Industrial Revolution.

Originally, Darwin's charted career called for him to become a doctor; an ambition that ended with his unfortunate loathing of blood. Instead, he obtained a degree in theology at the University of Cambridge, but ultimately pursued not a life in the church, but a life of science.

Darwin was an immensely curious person who conducted notorious chemistry experiments as a boy. He collected everything – beetles were a favorite – and read all he could find on science.

It's easy now to imagine Darwin as the detached, objective observer of life and nature, the perfect person to produce “On the Origins of Species,” the very model of a modern major scientist.

Atheists trumpet Darwin's atheism, but Darwin considered himself more of an agnostic. “Origins,” assert Desmond and Moore, is the natural product of Darwin's deep and subtle faith and sense of morality. He believed not just in a brotherhood of all humankind – a world without slavery – but extended that belief to all living things. Given that point of view, it was no leap of faith to conclude all life might, in fact, share a common heritage and ancestry.

Darwin, of course, knew these thoughts would be considered subversive, inflammatory and controversial by many of his contemporaries. He was loathe to attract their attention and amity. It's part of the reason why it took more than 20 years for him to write “Origins.” (It made its debut 150 years ago.)

Desmond and Moore lay out their arguments for Darwin's moral imperatives in rich, sometimes mind-numbing detail. Already well-steeped in their subject from producing a highly regarded 1991 biography, the authors reportedly pored through Darwin's family correspondence (thousands of letters), personal diaries, the logbooks of the HMS Beagle, Parliamentary papers, the literature Darwin read and the newspapers of his time to establish and explain why the driving force behind Darwin's theory of evolution was fundamentally his fierce hatred of slavery.

It's a compelling case, though one that will undoubtedly be debated by Darwin scholars. What is beyond dispute is the fact that Darwin, like Lincoln, profoundly altered the way we see ourselves. What we do with that information about his theory still remains to be seen.